Dimension 4 is somewhat difficult to interpret. It seems to be a measure of the extent to which two types of non-traditional household occur. On the one hand, there are areas with many "non-family" households and numerous young adults (coupled with high income, substantial levels of education, and high population density). On the other hand there are areas with many female-headed families and numerous children (these tend to be much poorer).
Nothing quite like dimension 4 appeared in the analysis for 1990. But note that dimension 4 is the least significant of the four dimensions and has a somewhat high correlation with dimension 2.